Re: [PATCH v9 3/8] KVM: Add KVM_EXIT_MEMORY_FAULT exit

Chao Peng
2022-11-18T01:37:32Z

On Thu, Nov 17, 2022 at 03:08:17PM +0000, Alex Bennée wrote:
>
> Chao Peng <chao.p.peng@linux.intel.com> writes:
>
> > On Wed, Nov 16, 2022 at 07:03:49PM +0000, Alex Bennée wrote:
> >>
> >> Chao Peng <chao.p.peng@linux.intel.com> writes:
> >>
> >> > On Tue, Nov 15, 2022 at 04:56:12PM +0000, Alex Bennée wrote:
> >> >>
> >> >> Chao Peng <chao.p.peng@linux.intel.com> writes:
> >> >>
> >> >> > This new KVM exit allows userspace to handle memory-related errors. It
> >> >> > indicates an error happens in KVM at guest memory range [gpa, gpa+size).
> >> >> > The flags includes additional information for userspace to handle the
> >> >> > error. Currently bit 0 is defined as 'private memory' where '1'
> >> >> > indicates error happens due to private memory access and '0' indicates
> >> >> > error happens due to shared memory access.
> >> >> >
> >> >> > When private memory is enabled, this new exit will be used for KVM to
> >> >> > exit to userspace for shared <-> private memory conversion in memory
> >> >> > encryption usage. In such usage, typically there are two kind of memory
> >> >> > conversions:
> >> >> > - explicit conversion: happens when guest explicitly calls into KVM
> >> >> > to map a range (as private or shared), KVM then exits to userspace
> >> >> > to perform the map/unmap operations.
> >> >> > - implicit conversion: happens in KVM page fault handler where KVM
> >> >> > exits to userspace for an implicit conversion when the page is in a
> >> >> > different state than requested (private or shared).
> >> >> >
> >> >> > Suggested-by: Sean Christopherson <seanjc@google.com>
> >> >> > Co-developed-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> >> >> > Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> >> >> > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> >> >> > ---
> >> >> > Documentation/virt/kvm/api.rst | 23 +++++++++++++++++++++++
> >> >> > include/uapi/linux/kvm.h | 9 +++++++++
> >> >> > 2 files changed, 32 insertions(+)
> >> >> >
> >> >> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> >> >> > index f3fa75649a78..975688912b8c 100644
> >> >> > --- a/Documentation/virt/kvm/api.rst
> >> >> > +++ b/Documentation/virt/kvm/api.rst
> >> >> > @@ -6537,6 +6537,29 @@ array field represents return values. The userspace should update the return
> >> >> > values of SBI call before resuming the VCPU. For more details on RISC-V SBI
> >> >> > spec refer, https://github.com/riscv/riscv-sbi-doc.
> >> >> >
> >> >> > +::
> >> >> > +
> >> >> > + /* KVM_EXIT_MEMORY_FAULT */
> >> >> > + struct {
> >> >> > + #define KVM_MEMORY_EXIT_FLAG_PRIVATE (1 << 0)
> >> >> > + __u32 flags;
> >> >> > + __u32 padding;
> >> >> > + __u64 gpa;
> >> >> > + __u64 size;
> >> >> > + } memory;
> >> >> > +
> >> >> > +If exit reason is KVM_EXIT_MEMORY_FAULT then it indicates that the VCPU has
> >> >> > +encountered a memory error which is not handled by KVM kernel module and
> >> >> > +userspace may choose to handle it. The 'flags' field indicates the memory
> >> >> > +properties of the exit.
> >> >> > +
> >> >> > + - KVM_MEMORY_EXIT_FLAG_PRIVATE - indicates the memory error is caused by
> >> >> > + private memory access when the bit is set. Otherwise the memory error is
> >> >> > + caused by shared memory access when the bit is clear.
> >> >>
> >> >> What does a shared memory access failure entail?
> >> >
> >> > In the context of confidential computing usages, guest can issue a
> >> > shared memory access while the memory is actually private from the host
> >> > point of view. This exit with bit 0 cleared gives userspace a chance to
> >> > convert the private memory to shared memory on host.
> >>
> >> I think this should be explicit rather than implied by the absence of
> >> another flag. Sean suggested you might want flags for RWX failures so
> >> maybe something like:
> >>
> >> KVM_MEMORY_EXIT_SHARED_FLAG_READ (1 << 0)
> >> KVM_MEMORY_EXIT_SHARED_FLAG_WRITE (1 << 1)
> >> KVM_MEMORY_EXIT_SHARED_FLAG_EXECUTE (1 << 2)
> >> KVM_MEMORY_EXIT_FLAG_PRIVATE (1 << 3)
> >
> > Yes, but I would not add 'SHARED' to RWX, they are not share memory
> > specific, private memory can also set them once introduced.
>
> OK so how about:
>
> KVM_MEMORY_EXIT_FLAG_READ (1 << 0)
> KVM_MEMORY_EXIT_FLAG_WRITE (1 << 1)
> KVM_MEMORY_EXIT_FLAG_EXECUTE (1 << 2)
> KVM_MEMORY_EXIT_FLAG_SHARED (1 << 3)
> KVM_MEMORY_EXIT_FLAG_PRIVATE (1 << 4)

We don't actually need a new bit, the opposite side of private is
shared, i.e. flags with KVM_MEMORY_EXIT_FLAG_PRIVATE cleared expresses
'shared'.

Chao
>
> >
> > Thanks,
> > Chao
> >>
> >> which would allow you to signal the various failure modes of the shared
> >> region, or that you had accessed private memory.
> >>
> >> >
> >> >>
> >> >> If you envision any other failure modes it might be worth making it
> >> >> explicit with additional flags.
> >> >
> >> > Sean mentioned some more usages[1][]2] other than the memory conversion
> >> > for confidential usage. But I would leave those flags being added in the
> >> > future after those usages being well discussed.
> >> >
> >> > [1] https://lkml.kernel.org/r/20200617230052.GB27751@linux.intel.com
> >> > [2] https://lore.kernel.org/all/YKxJLcg%2FWomPE422@google.com
> >> >
> >> >> I also wonder if a bitmask makes sense if
> >> >> there can only be one reason for a failure? Maybe all that is needed is
> >> >> a reason enum?
> >> >
> >> > Tough we only have one reason right now but we still want to leave room
> >> > for future extension. Enum can express a single value at once well but
> >> > bitmask makes it possible to express multiple orthogonal flags.
> >>
> >> I agree if multiple orthogonal failures can occur at once a bitmask is
> >> the right choice.
> >>
> >> >
> >> > Chao
> >> >>
> >> >> > +
> >> >> > +'gpa' and 'size' indicate the memory range the error occurs at. The userspace
> >> >> > +may handle the error and return to KVM to retry the previous memory access.
> >> >> > +
> >> >> > ::
> >> >> >
> >> >> > /* KVM_EXIT_NOTIFY */
> >> >> > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> >> >> > index f1ae45c10c94..fa60b032a405 100644
> >> >> > --- a/include/uapi/linux/kvm.h
> >> >> > +++ b/include/uapi/linux/kvm.h
> >> >> > @@ -300,6 +300,7 @@ struct kvm_xen_exit {
> >> >> > #define KVM_EXIT_RISCV_SBI 35
> >> >> > #define KVM_EXIT_RISCV_CSR 36
> >> >> > #define KVM_EXIT_NOTIFY 37
> >> >> > +#define KVM_EXIT_MEMORY_FAULT 38
> >> >> >
> >> >> > /* For KVM_EXIT_INTERNAL_ERROR */
> >> >> > /* Emulate instruction failed. */
> >> >> > @@ -538,6 +539,14 @@ struct kvm_run {
> >> >> > #define KVM_NOTIFY_CONTEXT_INVALID (1 << 0)
> >> >> > __u32 flags;
> >> >> > } notify;
> >> >> > + /* KVM_EXIT_MEMORY_FAULT */
> >> >> > + struct {
> >> >> > +#define KVM_MEMORY_EXIT_FLAG_PRIVATE (1 << 0)
> >> >> > + __u32 flags;
> >> >> > + __u32 padding;
> >> >> > + __u64 gpa;
> >> >> > + __u64 size;
> >> >> > + } memory;
> >> >> > /* Fix the size of the union. */
> >> >> > char padding[256];
> >> >> > };
> >> >>
> >> >>
> >> >> --
> >> >> Alex Bennée
> >>
> >>
> >> --
> >> Alex Bennée
>
>
> --
> Alex Bennée